1. Field of the Invention
This invention relates to a thermal mass flow meter that measures a mass flow rate of a fluid flowing through a piping based upon a temperature distribution of the fluid in a flowing direction of the fluid.
2. Description of the Related Art
Referring to FIGS. 5A and 5B, the following description will discuss a conventional thermal mass flow meter. FIG. 5A is a cross-sectional view and FIG. 5B is a graph that shows a temperature distribution on a surface of a piping. In FIG. 5B, the axis of abscissas represents the position in a flowing direction, and the axis of ordinates represents the temperature. A curved line, indicated as a broken line, represents the temperature distribution in a state with no fluid flowing through the piping, and a curved line, indicated by a solid line, represents the temperature distribution in a state with a fluid flowing through the piping.
As shown in FIG. 5A, a heater 32 is secured onto a surface of a peripheral face of a piping 31 in contact therewith, and paired temperature sensors 34 (34a, 34b), which are used for measuring a surface temperature of the piping 31, are placed at positions in the flowing direction of the piping 31 on an upstream side and a downstream side of the heater 32, with equal distance apart therefrom. In this example, a flow-rate measuring chip 36 in which the heater 32 and paired temperature sensors 34 are assembled in a single substrate by using a MEMS (Micro Electro Mechanical System) technique, and the flow-rate measuring chip 36 is attached to the piping 31 so that the flow rate in the piping 31 can be measured (for example, see U.S. Pat. No. 6,813,944).
In the thermal mass flow meter, the fluid inside the piping is heated to a predetermined temperature by the heater 32, and the surface temperatures of the pipe 31 at the respective positions are measured by the temperature sensors 34a and 34b. The temperature distribution of the fluid heated by the heater 32 comes to have virtually a Gaussian distribution; therefore, when the fluid stands still, the temperatures detected by the two temperature sensors 34a and 34b are equal to each other as indicated by a broken line in FIG. 5B, with the result that the measured temperature difference between the two positions becomes zero. When the fluid flows through the piping 31, the temperature distribution is shifted toward the downstream side and changed as indicated by a solid line in FIG. 5B so that the temperatures detected by the temperature sensors 34a and 34b have a difference. The temperature distribution of the surface of the piping 31 is shifted toward the downstream side as the flow rate of the fluid flowing through the piping 31 increases; therefore, when the apex of the temperature distribution of the surface of the piping 31 is located between the temperature sensors 34a and 34b, the difference in measured temperatures has a greater value as the flow rate of the fluid flowing through the piping 31 increases. In this manner, since there is a correlation between the flow rate of the fluid flowing through the piping 31 and the difference in measured temperatures between the temperature sensors 34a and 34b, the flow rate of the fluid flowing through the piping 31 can be calculated based upon the difference in measured temperatures between the temperature sensors 34a and 34b by utilizing the correlation.
In this thermal mass flow meter using the chip 36 for use in measuring the flow rate in which the heater 32 and the paired temperature sensors 34a and 34b are assembled together with each other, the paired temperature sensors 34 can be placed in the vicinity of the heater by using the MEMS technique. Consequently, even in the case when the amount of transfer of the temperature distribution is small, since the temperatures of the temperature sensors 34a and 34b can be measured at positions, each having an abrupt inclination, of the curved line (see FIG. 5B) indicating the temperature distribution of the temperature sensor chip, it becomes possible to obtain a greater value as a measured temperature difference even upon having a fine amount of flow rate, and consequently to carry out a flow-rate measuring process with high sensitivity.
However, the process for assembling the heat generating element 32 and the paired temperature sensors 34 onto a single substrate by using the MEMS technique requires expensive manufacturing facilities, and can not be achieved at low costs. For this reason, the inventors of the present invention have proposed a structure in which a chip-type heater chip serving as a heat-generating element and paired temperature sensors, manufactured separately from the heater chip, are independently placed on the peripheral face of the piping so that a thermal mass flow meter is formed. With this structure, since the MEMS technique is no longer required, it becomes possible to manufacture the thermal flow meter at low costs.